Various dyes are known to the art that are infrared (IR) absorbers and which can act as photothermal sensitizers for various reactions that are initiated using near IR radiation. Examples include the SQS and related squarylium dyes disclosed in U.S. Pat. No. 5,019,549 which are useful dyes for use in donor elements for laser-induced thermal imaging processes.
The hexaarylbiimidazoles (HABIs) are a very useful class of photoinitiators that are widely known to those skilled in the art and which are used in many photopolymer products. Some key HABI review articles include the following: “Photoinitiators for Free-Radical-Initiated Photoimaging Systems”, B. Monroe and G. Weed, Chem. Rev., 93, p. 435-448 (1993), see p. 440-441 for the section on HABIs; and “Photopolymers: Radiation Curable Imaging Systems” by B. M. Monroe, Chapter 10 in Radiation Curing: Science and Technology, edited by S. P. Pappas, Plenum Press, New York, (1992), see p. 407-412 for the section on HABIs.
There are various compounds known that are sensitizers of HABIs for photopolymer processes effected with actinic radiation in the ultraviolet and visible regions of the electromagnetic spectrum. It would be very desirable to have efficient sensitizers of HABIs for photopolymer processes in the near infrared (˜770-1500 nm) of the electromagnetic spectrum, particularly since this type of actinic radiation is readily available from inexpensive diode lasers that can be employed for direct digital imaging. Efficient sensitizers of HABIs active in the near infrared are not currently known to the art, and there is a strong need to have efficient sensitizers of HABIs with sensitization in the near infrared region of the electromagnetic spectrum.
In Japanese patent publication 08-297364, there are disclosed some near infrared cationic dyes present in photosensitive image-forming sheets that sensitize HABIs. All of these dyes though have a borate anion, which is a well-known coinitiator of HABI(s), present as counterion. Since borate anion is present in these compositions, the dyes would sensitize HABIs. Having a borate anion present as counterion can be disadvantageous, since this anion is relatively large tending to impart limited solubility to the dye and hence likely making it difficult to tailor properties of compositions comprising these dyes. These compositions are comprised of three essential components—near IR dye, borate anion, and HABI. A two component composition—near IR dye and HABI—of the present invention obviates the need for the large borate anion.